Our overall objective is to understand the mechanisms of signal processing in the auditory nervous system. The cat cochlear nucleus provides a model system for studying how sensory nerve inputs produce output discharges in second order neurons. In a general sense, the function of the cochlear nucleus is to receive incoming auditory nerve discharges, modify the message and distribute the resulting output signals to higher centers in the brain. How auditory information is subsequently processed by the brain will be heavily dependent on events in the cochlear nucleus. Because functional properties must ultimately be based on anatomy, we have developed techniques whereby single auditory nerve fibers can first be electrophysiologically characterized by recording with micropipettes inserted into the axon, and then be labelled by intracellular injections of horseradish peroxidase (HRP) through the same pipettes. After histological processing, each labelled neuron can be reconstructed from serial sections from its peripheral ending in the cochlea to its central ramifications in the cochlear nucleus. Since the HRP reaction product is electron dense, these identified neurons can ultimately be examined with electron microscopy in order to determine the nature of their synaptic connections. These methods for staining and studying single neurons after characterizing their physiological response properties will allow us to describe structure-function relationships at the cellular level. The compilation of these data should help to generate a new level of understanding for mechanisms of signal processing, and will set the stage for studying the consequences of cochlear and central pathology on the cochlear nucleus. The possibility of practical prosthetic devices that successfully bypass nonfunctioning cochleas should insure continued interest in studies of stimulus coding at the level of the auditory nerve and cochlear nucleus.